Symmetric Tree

Posted 2021-01-21 ruruozhenhao

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   /   2   2
 /  / 3  4 4  3

 

But the following [1,2,2,null,3,null,3] is not:

    1
   /   2   2
         3    3

 

Note:
Bonus points if you could solve it both recursively and iteratively.

 

Approach #1: C++.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        
        Stack<TreeNode> stack = new Stack<TreeNode>();
        
    }
}